1. Field of the Invention
The present invention relates to a coordinate input apparatus and, more particularly, to a coordinate input apparatus in which a vibration input from a vibration pen is detected by a plurality of vibration sensors arranged on a vibration transmission plate, thereby detecting coordinates of the vibration pen on the vibration transmission plate.
2. Related Background Art
The above coordinate detecting system has advantages that an arrangement of the vibration transmission plate constituting an input tablet is simple and the vibration transmission plate can be placed on a display unit or an original because a transparent member can be utilized as the vibration transmission plate.
In a coordinate input apparatus of this type, an input vibration from the vibration pen is detected by a plurality of vibration sensors arranged on the vibration transmission plate Correct coordinates cannot be detected, however, unless influences of a phase delay and a circuit delay time of a detector upon vibration transmission on the vibration transmission plate are corrected. The circuit delay includes not only a delay of an electrical circuit but also that of a circuit between the vibration pen and the vibration transmission plate through which a vibration is mechanically transmitted
FIG. 8 is a graph in which the abscissa represents a time t and the ordinate represents a vibration transmission distance from an input point to a vibration sensor. FIG. 8 shows changes in group delay time tg required for an envelope of a vibration waveform to propagate for a certain distance and in a phase delay time tp required for the phase of the waveform to propagate for the distance
As shown in FIG. 8, a circuit delay time et is included even when the distance is 0, thereby offsetting a curve (straight line) representing individual delay times to the right in the graph. A phase waveform generates regular phase delays in accordance with a wavelength as shown in FIG. 8. A difference tof between the group delay time tg and the phase delay time tp at distance 0, i.e., a phase offset changes in accordance with the circuit delay time.
In a system in which a transverse component on a vibration transmission plate is detected to measure a vibration transmission time, a method of determining the vibration transmission time by using both the group and phase delay times tg and tp is known In this method, however, a correct vibration transmission time cannot be obtained unless correction of the circuit delay time et and the phase offset tof is performed.
For this reason, in a known conventional method, a correction value corresponding to average circuit delay time et and average phase offset tof measured in advance is stored and subtracted from a measurement value to correct the vibration transmission time.
Upon determination of the circuit delay time et and the phase offset tof, however, a calculation must be performed for data sampled at a certain point by using constants such as a group speed, a phase speed and a period. Therefore, coordinate determination precision is sometimes degraded by a speed error or a calculation error.
In addition, an actual circuit delay time changes in accordance with an environmental variation such as a temperature change. Therefore, in the conventional system, a correct vibration transmission time cannot be measured when environmental conditions change.